1. Field of the Invention
This invention relates to dot printers, and more particularly to the structure of a printing head thereof.
2. Description of the Prior Art
FIGS. 1 and 2 show a first conventional example of support structure for needles in the top end of the printing head of a dot printer. Reference numeral 1 designates a guide holder. To the front surface of the guide holder 1 on the platen side is fixed a needle guide 3 which holds a plurality of needles 2 in a slightly projected state. During printing operations the needles 2 are driven by an electromagnet and strikes against a platen. However, the needles 2 are so thin that they may push through a printing ribbon 4. When the needles 2 are densely arranged, as is necessary for the printing of characters Chinese, the top ends of the needles 2 are as thin as 0.2-0.25 mm in diameter, and are accordingly liable to push through. Since the needles 2 project from the front surface of the needle guide 3 even during the non-excited states of their respective electromagnets, the printing ribbon 4 still catches on the needles 2 as shown in FIG. 2. The printing ribbon 4 may be moved to the side while a needle is so caught. Therefore, the needles 2 can be subjected to side pressure, resulting in the bending or breaking of the needles 2.
In a second example shown in FIGS. 3 and 4, the front end of the needles 2 and the front surface of the needle guide 3 coincide. In this arrangement when a needle 2 is fully retracted, it would seem that the printing ribbon 4 is separated from the needle 2. However, a certain time is requred for the needle 2 to return completely (corresponding to T in FIGS. 9 amd 10), and the needle 2 before returning is subjected to side pressure from the ribbon 4 being moved to the side. Therefore, the danger of bending or breaking a needle 2 is not eliminated. Moreover, in the arrangement as shown in FIG. 3, if the top end of a neelde 2 is even slightly thickened by striking against a platen, the needle 2 sticks to its support hole 5 in the needle guide 3 and cannot slide. In this situation, abrasion in the armature of the corresponding electromagnet and on the rear end of the needle 2 also becomes serious.
In a third example as shown in FIG. 5, a recess 6 is formed in the front of the needle guide 3 by means of cutting work, and the needles 2 project from the bottom of the recess 6. In this arrangement, even if the top end of a needle 2 becomes thick because of striking against a platen, the needle 2 does not stick to the support hole 5 of the needle guide 3. However, if the needle 2 pushes through a printing ribbon 4, the printing ribbon 4 can catch on the needle 2 during restoring and be drawn into the bottom of the broad recess 6. If this happens, phenomena similar to the case shown in FIGS. 1 and 2 occur. Individual needles 2 differ from each other in frequency of striking against the platen. Therefore, unevenness of the top end of the needle 2 occurs. In the arrangement shown in FIG. 3 and FIG. 5, the top end of the needle 2 cannot be ground in order to correct the above mentioned unevenness. Because the front surface of the needle guide 3 and the top end surface of the needles 2 coincide, an artificial ruby or sapphire is used in the needle guide 3 for improving abrasion resistance.